Surface and groundwaters become contaminated with dyes due to discharge into the environment, which increases the risk of a number of human diseases. Many methods of dye removal from discharge waters at the source have been developed, but few are effective and the most effective method (activated carbon) is very expensive. The purpose of the present study was to test a natural zeolite (clinoptilolite type) as a potentially effective and inexpensive method to remediate dye discharge into the environment. In the removal experiments, malachite green (MG) and rhodamine B (RB) cationic dyes were used. The effects of various experimental conditions such as initial dye concentration, pH, and temperature on dye removal were investigated in a single-dye system. The degree of removal of MG and RB increased with increasing initial concentration and temperature of the dye in a single-dye system. An increase in pH decreased RB removal, but increased MG removal. In a two-dye system, MG and RB adsorption decreased by ~41.74 and 21.51%, respectively, due to competitive adsorption of the two dyes. Adsorption reflected a pseudo-second order kinetics model with high correlation coefficients (r2 = 0.996–1.000) in single-dye and two-dye systems. Adsorption was most consistent with the Langmuir-1 and the Redlich-Peterson isotherm models with high correlation coefficients (r2 = 0.987–0.999) in both systems. The Langmuir-1 adsorption capacities were determined as 43.86 and 44.25 mg/g for the removal of MG and RB in single-dye systems, respectively. In a two-dye system, the Langmuir-1 capacities were 20.62 and 31.54 mg/g for the removal of MG and RB, respectively.

The Pleistocene sediment infill of elongated glacial incisions of the southern North Sea (SNS) is often referred to as tunnel valleys (TVs). The depositional environment is not yet fully understood and the present study addresses this challenge from the perspective of clay-mineral transformation (illite to illite-smectite) reported from the largest Elsterian TV of the SNS. Material acquired from the K14-12 borehole in the Dutch offshore was analyzed by X-ray diffraction (XRD), electron microscopy, electron microprobe, and laser particle-size analysis. Illite and illite-smectite (I-S) appeared as dominant clays along with minor amounts of kaolinite, kaolinite-smectite, and chlorite. The largest amount of I-S is recognized in the main TV portion, while in pre-glacial and uppermost deposits, I-S is less abundant. The XRD peak fitting and deconvolution suggest that I-S consists of several intermediates — ordered (well crystallized illite + R3 I-S) and disordered (R0 I-S + R0 I-SS). Given the average particle sizes (>2 µm) and Kübler index values (0.415–0.341°Δ2θ), illite as well as chlorite and kaolinite were interpreted as detrital. On the basis of the distinctive distribution, grain sizes, and compositional variations of I-S, formation by means of early diagenetic in situ smectitization of illite under a cold climate is proposed. The process operated via a series of mixed-layer intermediates derived from an illite component being converted progressively to low-charged smectite. The reaction is marked by a significant net loss of K and Al with replacement by Si in a tetrahedral coordination. Layer-charge imbalance is accommodated by Fe(III) and Mg entering an octahedral sheet, whereas Ca partly fills the interlayer sites. Smectitization rates were controlled by illite grain sizes. The results of the present study support strongly the existence of an ice-marginal freshwater depositional environment at the glacial maximum in the SNS in which early diagenesis at low temperatures resulted in incomplete conversion of illite to smectite.

Three kaolinite reference samples identified as KGa-1, KGa-1b, and KGa-2 from the Source Clays Repository of The Clay Mineral Society (CMS) are used widely in diverse fields, but the defect structures have still not been determined with certainty. To solve this problem, powder diffraction patterns of the KGa-1, KGa-1b, and KGa-2 samples were modeled. In a kaolinite layer among three symmetrically independent octahedral sites named as A, B, and C and separated from each other by b/3 along the b parameter, the A and B sites are occupied by Al cations, whereas, the C sites located along the long diagonal of the oblique kaolinite unit cell are vacant. The layer displacement vectors t1 and t2 are related by a pseudo-mirror plane from defect-free 1Tc kaolinite enantiomorphs, whereas, the random interstratification within individual kaolinite crystallites creates right-hand and left-hand layer sub-sequences producing structural disorder. A third layer displacement vector, t0, located along the long diagonal of the oblique layer unit cell that contains the vacant octahedral site and coincides with the layer pseudo-mirror plane may exist. Thus, a structural model should be defined by the probability of t1, t2, and t0 layer displacement translations Wt1, Wt2, and Wt0, respectively, determined by simulated experimental X-ray diffraction (XRD) patterns. X-ray diffraction patterns were calculated for structures with a given content of randomly interstratified displacement vectors, and other XRD patterns were calculated for a physical mixture of crystallites having contrasting structural order with only C-vacant layers. The samples differ from each other by the content of high- and low-ordered phases referred to as HOK and LOK. The HOK phase has an almost defect-free structure in which 97% of the layer pairs are related by just the layer displacement vector t1 and only 3% of the layer pairs form the enantiomorphic fragments. In contrast, the LOK phases in the KGa-1, KGa-1b, and KGa-2 samples differ from HOK phases by the occurrence probabilities for the t1, t2, and t0 layer displacements. In addition, the LOK phases contain stacking faults that displace adjacent layers in arbitrary lengths and directions. Low XRD profile factors (Rp = 8-11%) support the defect structure models. Additional structural defects and previously published models are discussed.

Sorption of U(VI) on clay and related minerals has been inspected experimentally and computationally because of its central role in safety considerations of geological repositories for highly radioactive waste. Np(V), which also has long half-life isotopes, has received considerably less attention. The purpose of the present study was to investigate computationally the adsorption of Np(V) on a clay-mineral surface and to compare it to adsorption of U(VI). As a sample case study, adsorption of Np(V) at the (110) edge surface of the common clay mineral montmorillonite was modeled. The density functional approach and periodic supercell models were applied. Mono- and bidentate adsorption complexes with coordination numbers 4 and 5 were inspected and compared to corresponding U(VI) species. While U(VI) prefers bidentate adsorption complexes with varying coordination numbers, Np(V) is more stable when monodentate-coordinated with a coordination number of four. In line with its smaller hydrolysis constant in aqueous solution, Np(V) shows a lower tendency to form monohydroxides on the mineral surface compared to U(VI). As no experimental geometry parameters are available for Np(V) adsorbed on montmorillonite, the results were compared tentatively to EXAFS data for adsorption at kaolinite and good agreement for the geometry changes due to adsorption was found for the more preferred adsorbed species.

Experimental studies have shown that a sharp, high-frequency IR band at ~3615 cm-1 (in H2O form) and at ~2685 cm-1 (in D2O form) is a common feature for all smectites, and its position correlates with layer charge. In order to explain the molecular origin of this band in terms of total layer charge, charge localization, as well as nature of interlayer cations influencing the position and intensity of this peak, a series of classical molecular dynamics (MD) simulations was performed for several smectite models. The smectite layers were described using a modified CLAYFF force field, where the intramolecular vibrations of H2O were described more accurately by the Toukan-Rahman potential. The power spectra of molecular vibrations of hydrogens were calculated for selected sub-sets of interlayer H2O to analyze quantitatively their contribution to the observed spectral features. The statistics of hydrogen bonds in the smectite interlayers were also analyzed to support the spectral calculations.

The simulation results demonstrated clearly that only the H2O molecules in close proximity to the smectite surface are responsible for the sharp vibrational band observed. Other hypotheses for the possible origins of this band were considered carefully and eventually rejected. Two orientations of H2O molecules donating one or two H bonds to the basal oxygens of the smectite surface (monodentate and bidentate orientations, respectively) were observed. In both orientations, these H bonds are quite weak, pointing to a generally hydrophobic character of the smectite surface. Both orientations contributed to the high-frequency band, but the monodentate orientation provided the predominant contribution because surface H2O molecules in this orientation were much more abundant. In good agreement with experiment, only a small difference in the peak position was observed between smectites with different charge localization. The effect of the total layer charge, i.e. the red-shift for higher-charge smectites, was also confirmed. This shift arose from the decrease in the H-bonding distances of H2O in monodentate and bidentate orientation.

Humic acid (HA)-clay complexes are well known for their contribution to soil structure and environmental processes. Despite extensive research, the mechanisms governing HA adsorption are yet to be resolved. A systematic study was conducted to characterize the adsorption of a soil-derived HA to seven clay minerals. Clay surfaces affected HA adsorption directly due to structural differences and indirectly by altering solution pH. The following order of HA removal was obtained for the clay minerals at their natural pH: illite ≫ palygorskite > kaolinite > sepiolite > montmorillonite = hectorite ≫ talc. Removal of HA (precipitation and adsorption) by kaolinite and illite was attributed to the low pH they induce, resulting in protonation of the clay and HA surfaces. In spite of the low pH, the zeta potential for HA remained negative, which promoted HA adsorption to the protonated clay surfaces by ligand exchange. Ionic strength did not affect HA adsorption to clay minerals with low zeta potentials, indicating that charge screening is not a major mechanism of HA adsorption for these minerals, and supporting the suggestion that ligand exchange is the main adsorption mechanism to pH-dependent sites. The increase in ionic strength did, however, promote HA adsorption to clay minerals with high zeta potentials. At pH 8–9 the order of HA affinity for clay minerals was: palygorskite >>sepiolite > montmorillonite = hectorite > kaolinite > illite > talc, emphasizing strong HA interactions with the fibrous clays. This strong affinity was attributed to their large surface areas and to strong interactions with OH groups on these clay surfaces. Results indicated that HA did not enter the intracrystalline channels of the fibrous clays but suggested that their macro-fiber structure facilitates HA adsorption. The sorption of HA to kaolinite further increased in the presence of Cu2+, and the sorption of Cu2+ increased in the presence of HA, due to a number of synergistic effects. This study emphasizes the diverse effects of clay structure and solution chemistry on HA adsorption.

Tropical soils range from nutrient-depleted lateritic soils rich in halloysite or kaolinite to Inceptisols rich in interstratified kaolinite-smectite (K-S), smectite, or related 2:1 clays. Given the strong influence of clay minerals on tropical soil quality, better understanding of factors influencing their occurrence is important for modeling and managing tropical environments. This study examines the alteration of smectite to kaolinite by way of intermediate K-S and halloysite in a 120 ka moist tropical chronosequence. Iron-rich smectite (11.6 ± 2.2% Fe2O3) is the dominant mineral in Holocene soils (1–8 ka) originating from sediments rich in plagioclase and clinopyroxene. The cation exchange capacity (CEC) of smectite is 54–84 cmolc/kg and pH is 6.1 to 7.4. Within 50 ka, smectite fixes Al-hydroxy complexes into interlayers, K+ is retained preferentially over Ca2+, and 2:1 layers are stripped of tetrahedral sheets; the resulting K-S inherits flaky smectite crystal habit and the 2:1 layers — which only expand partially — include Al-hydroxy smectite and some illite-like layers. After 50 ka, the dominant mineral is K-S, the CEC is 18–28 cmolc/kg, and the pH is 5.3. Flaky Fe-kaolinite with ~10% residual smectite layers and halloysite (7.4% Fe2O3) also occur in 50 ka soil. The 120 ka soils are dominated by flaky Fe-kaolinite (<10% residual smectite layers) and halloysite (4.9% Fe2O3), and Fe-poor hexagonal kaolinite also occurs (5–10% of soil). The CEC is 11–16 cmolc/kg and the pH is 4.7–5.3.

Changes in crystal chemistry of the soil clays (decreasing Fe, Mg, Ca, and K; increasing Al) over time reflects two reaction mechanisms: (1) cell-preserved transformation of smectite layers to kaolinite layers that accompanies conversion of smectite to K-S and eventually kaolinite; this results in the formation of flaky Fe-rich kaolinites after 50 ka; and (2) dissolution of K-S followed by crystallization of halloysite. Neoformation of hexagonal kaolinite and/or halloysite with low Fe (<3% Fe2O3) follows dissolution of Fe-kaolinite or halloysite after 100 ka. This sequence is probably common in moist tropical soils and these findings may inform modeling of soil composition in tropical landscapes where tectonic, volcanic, or geomorphic activity periodically exposes unweathered parent material, producing a range of soil ages.

Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form:

K+ > Na+ > Ca2+ > Cs+ > Ba2+

The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide-soil interactions, and enhanced oil recovery.

Organo-clays represent a special challenge for molecular simulations because they require accurate representation of the clay and the organic/aqueous sections of the model system and accurate representation of the interactions between them. Due to the broad range of force-field models available, an important question to ask is which sets of parameters will best suit the molecular modeling of the organo-intercalated smectites? To answer this question, the structure of the ethylene glycol (EG)-smectite complex is used here as a testing model because the intercalation of EG in smectites provides a stable interlayer complex with relatively constant basal spacing.

Three smectite samples with substantially different layer charge and charge localization were selected for X-ray diffraction (XRD) measurements. Their molecular models were built and molecular-dynamics simulations performed using various combinations of the organic force fields (CGenFF, GAFF, CVFF, and OPLS-aa) with ClayFF and INTERFACE force fields used to describe smectites. The simulations covered a range of different EG and water contents. For every structure, the density distribution of interlayer species along the direction perpendicular to the layer plane was calculated and then used to optimize the XRD patterns for these simulated models.

A comparison of these results with experimental XRD patterns shows very large discrepancies in the structures and basal spacings obtained for different layer charges as well as for different force fields and their combinations. The most significant factor affecting the accuracy of the calculated XRD patterns was the selection of the clay-mineral force-field parameters. The second important conclusion is that a slight modification of the basal oxygen parameters for non-electrostatic interactions (increase of their effective atomic diameters) may be a simple and straightforward way to improve significantly the agreement between the modeled XRD patterns with experiments, especially for high-charge smectites. Generally, among organic force fields, the least accurate results were obtained with CGenFF. For unmodified ClayFF, its combination with GAFF gave the best results, while the two other sets (OPLS-aa and CVFF) gave the best results in combination with ClayFFmod. The INTERFACE and INTERFACEmod produced much better results for low-charge montmorillonite than for high-charge smectites.

The study of hard rock conversion into fine earths and clayey materials in the pedosphere is important in understanding the relative proportions of recent soil features to features that were inherited from ancient epochs. Cold environments are widely thought to be areas of physical weathering, but the coexistence of physical and chemical processes have also been shown. To further examine mafic rock (dolerite) weathering in soil environments and the conversion into clayey materials, Entic Podzols formed in the cold continental climate were studied. The key study was located in the central part of the flood basalt complex, or traps (traprocks), of the Central Siberian Plateau (Russia). The qualitative mineralogy was studied using X-ray diffraction and the quantitative mineral composition was determined using X-ray diffraction and subsequent Rietveld analysis. The micromorphological characteristics of the soils were studied in thin sections. Dolerite fragments and fine earths were sampled from soil profiles underlain by dolerite. XRD analyses indicated that pyroxene and especially plagioclase contents in the dolerite fragments and fine earths decreased from the bottom to the top soil horizons mostly in the mature soil profiles that were affected by chemical weathering of dolerite. The dioctahedral and trioctahedral smectites in the soils were inherited from a dolerite previously subjected to chemical weathering. The smectite was conserved in the inherited aggregates and protected against dissolution even in acidic soil horizons. Recent pedogenesis processes fractured individual fragments, converted it into soil micromass, and slightly decreased the total smectite content of the <1 µm soil fraction. However, in soil samples collected from the bottom to the top horizons of a mature soil profile, trioctahedral smectite contents decreased as dioctahedral smectite contents increased. This suggests that dioctahedral smectites formed by pedogenic alteration of inherited trioctahedral smectites.

Land colonization with vascular plants during the late Silurian—early Devonian and then arborescence during the Pragian—Givetian caused the development of new soil types. These true-rooted soils increased the rate of pedogenesis on a global scale. Since that time, soil has become a key component of the biosphere and has given rise to profound development of the Earth’s Critical Zone (CZ). Case studies of Devonian CZs have helped to record the transformation from Precambrian—Lower Paleozoic ‘proto-CZs,’ which had insufficient proto-soil cover, to modern soils with true-rooted pedosphere. Devonian (Givetian—Frasnian) paleosols from the Voronezh region of Russia are combined into pedocomplexes which occupied the top, slope, and bottom positions of a pronounced paleo-relief. Paleosols were developed from terrigenous argillites and volcanigenic-sedimentary deposits. Each pedocomplex consisted of four or more paleosols with different degrees of development and profile preservation. Paleosols exhibited several common characteristics including production and translocation of clay, ferruginization and the presence of siderite nodules, enhanced MnO/Al2O3 and (Fe2O{3}+MnO)/Al2O3 values, and in situ roots and root-system traces. The latter are siderite/goethite substituted. Stable isotope analysis of siderite shows δ13C values of between -6.1 and -13.7% indicating that CO2 had originated from C3 plants. The main mineral component of clay fractions in automorphic paleosols (top and slope of the paleorelief) is kaolinite. The important feature of these paleosols is the red-stained hematite-rich layer in their bases. These horizons developed at different depths and with different thicknesses, and marked the paleo-groundwater tables of each sub-CZ. Evidence of the imprints of vegetation is seen in the abundant in situ roots, plant fragments, and spores of rhyniophytes, lycopsids, progymnosperms, cladoxylalean ferns, and phytoleims of algae-like vascular plants. The near-equatorial location and the overall characteristics of paleosols studied suggest that the aforementioned horizons were formed in a tropically warm and humid climate. The paleo-ecological environments which accompanied pedogenesis were probably controlled by tectonic activity and volcanism.

Interlayer swelling of hydrated montmorillonite is an important issue in clay mineralogy. Although the swelling behavior of montmorillonite under ambient conditions has been investigated comprehensively, the effects of basin conditions on the hydration and swelling behaviors of montmorillonite have not been characterized thoroughly. In the present study, molecular dynamics simulations were employed to reveal the swelling behavior and changes in the interlayer structure of Na-montmorillonite under the high temperatures and pressures of basin conditions. According to the calculation of the immersion energy, the monolayer hydrate becomes more stable than the bilayer hydrate at a burial depth of 7 km (at a temperature of 518 K and a lithostatic pressure of 1.04 kbar). With increasing burial depth, the basal spacings of the monolayer and bilayer hydrates change to varying degrees. The density-distribution profiles of interlayer species exhibit variation in the hydrate structures due to temperature and pressure change, especially in the structures of bilayer hydrate. With increasing depth, more Na+ ions prefer to distribute closer to the clay layers. The mobility of interlayer water and ions increases with increasing temperature, while increasing pressure caused the mobility of these ions to decrease.